Preconditioner for metallizing of polyolefins

ABSTRACT

A PROCESS OF MELTALLIZING MOLDED POLYOLEFIN ARTICLES, ESPECIALLY THOSE MADE OF POLYPROPYLENE, WHEREIN THE ARTICLE IS PRETREATED WITH A CONDITIONER COMPRISING A KETONE WHICH IS CAPABLE OF PENETRATING THE SURFACE OF THE POLYOLEFIN ARTICLE AND WHICH IS DISPERSIBLE, EMULSIFIABLE OR AT LEAST SLIGHTLY SOLUBLE IN WATER. ACETONE AND METHYL ETHYL KETONE ARE PARTICULARLY USEFUL KETONES FOR USE AS THE POLYOLEFIN PRECONDITIONER.

States Patent @ffice P 3,649,476 atented Mar. 14, 1972 3,649,476 PRECONDITIONER FOR METALLIZING F POLYOLEFINS Habet M. Khelghatian and Wassily W. Poppe, Springfield,

Pa., and James E. Fitzpatrick, New Castle, Del., assignors to Standard Oil Company, Chicago, Ill. No Drawing. Filed Oct. 11, 1968, Ser. No. 766,996 Int. C1. C23!) /60; B44d 1/092 US. Cl. 204-20 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND This invention relates to a method of preconditioning the surfaces of polyolefin articles, particularly polypropylene articles, prior to the deposition thereon of metallic coatings. More particularly, this invention relates to a method of treating the surface of polyolefin articles to render them more easily amenable to deposition of metallic coating thereon, eg by electroplating or vacuum metallizing. Still more particularly, this invention relates to the substantial elimination of surface strains in molded polypropylene articles which cause problems in subsequent deposition of metallic coatings.

The problem of causing ink, paint, or other types of coating to adhere to the non-polar surface of polyolefins is well known. In general, an approach to solving the problem has been to modify the surface of the polyolefin by surface oxidation to introduce polar groups onto the surface. For example, in the case of electroplating, one may treat the polyolefin article with a strong mineral acid containing chromium trioxide or an alkali metal chromate. The mineral acid used should be a strong one capable of oxidizing the polyolefin surface. Concentrated sulfuric acid, phosphoric acid, or mixtures thereof have been used with high degrees of success. The purpose of this treatment is to induce surface oxidation. After the acid treatment, the surface of the article is sensitized with a solution of a readily oxidizable salt such as stannous chloride. The polyolefin article is then treated with an aqueous solution of a noble metal salt (for example, palladium chloride) which is reduced by the oxidizabe salt present on the surface. This forms a metallic film at discrete activated sites. The activated material is then subjected to electroless plating using copper, nickel or cobalt as the metal. This may be accomplished by immersing the treated surface in a metal solution which also contains a reducing agent such as formaldehyde, trioxymethylene or the like. Suflicient metal is thereby deposited on the surface to form a continuous metallic film capable of conducting electricity. The polyolefin article may then be conventionally electroplated with copper, followed by nickel or chromium, or just with nickel followed by chromium. The thickness of the metal coatings is generally within the range of about 0.1 to 1.5 mils. In this method it is desirable to rinse the surface with water following each of the above-mentioned steps.

The electroplating of polyolefin articles with a metal plate having a thickness in the range of about 0.1 to 1.5 mils in such a way that the plate is tightly bonded to the polyolefin substrate constitutes a highly desirable goal.

This is especially true in view of recently developed polyolefins which are considered engineering plastics and which may be used as substitutes for various materials of construction. An electroplated metal coating having good adhesion to the plastic substrate improves the structural properties of the plastic, such as resistance to deformation, thereby enhancing its use as a substitute for metals. The advantages of using polyolefins in such applications include lower cost of materials, cheaper tooling and tool maintenance, lower finishing costs in buffing and polishing, and lower shipping costs. The use of polyolefins, furthermore, allows greater versatility of product design and gives a more corrosion-resistant end product. Good adhesion between the metal plate and the polyolefin substrate causes improvement in physical properties such as flexural modulus, impact strength, and temperature deflection.

Another method of applying a metallic coating to a nonconductive polyolefin article is vacuum metallizing. This well known and conventional procedure involves the principle of evaporation of metals under high vacuum. Representative of typical metals that may be applied using this technique include aluminum, copper, and silver. Generally, vacuum metallizing involves the steps of (a) applying a suitable undercoat or prime coat to the polyolefin article, (b) evaporating the desired metal under high vacuum, and (c) applying a topcoat lacquer to protect the thin metallic deposit. Suitable undercoats that are applied to the polyolefin article are well known and are generally a dispersion or solution of an acid containing polymer such as carboxylated butadiene polymers, and maleic anhydride modified atactic polypropylene polymers. As the topcoat, any commercially available thermosetting acrylic lacquer may be used.

It is well known that when molding polyolefin articles, especially into complex shapes, strains occur within the material and these strains cause poor results when applying metallic coatings to the polyolefin material. Therefore, it is desirable to eliminate these strains prior to depositing metallic coatings upon the molded surfaces. It is an object of this invention to improve the quality of metallic coatings applied to molded polyolefin articles. Another object of this invention is to prepare the polyolefin material for surface conditioning or etching prior to plating the surface with metals. A further object of this invention is to relieve the strains in the polyolefin produced by the molding procedure, prior to conditioning the polyolefin for electrodeposition of a metal coating. Specially, the object of this invention is to precondition articles formed from commercially available polyolefins and in particular polypropylene, in order to render them metallizable by standard means applicable to the commercial plating of other plastics.

SUMMARY OF THE INVENTION We have found that the foregoing objects may be obtained by contacting the article to be plated in accordance with the above-described procedure with certain ketones at a temperature between about 20 C. and the boiling point of the ketone for a period of from about 1 minute to about 15 minutes prior to oxidizing the polyolefin surface or treating the surface with a primer coating. Higher temperatures enable reduced contact time and therefore the time may be less than one minute. Contact time in excess of 15 minutes is not considered practical. Preferred ketones to be used with this invention have a boiling point less than about 0., should be at least slightly soluble in water or dispersible or emulsifiable in water, and should be capable of penetrating the surface of the polyolefin article. Ketones having these properties include acetone, methyl ethyl ketone, methyl isopropyl ketone, methyl isobutyl ketone and ethyl t-butyl ketone. The ketone should be at least partially removed from the surface of the polyolefin article by washing with water prior to the mineral acid treatment. The contacting of the article with the ketone may take place either in the liquid or vapor state of the ketone. Methyl ethyl ketone and acetone are particularly suitable for pretreating articles which have been molded from polypropylene.

After such pretreatment the polyolefin, if electroplating is contemplated, is conditioned by treatment with (l) a strong mineral acid containing chromium trioxide or an alkali metal chromate, (2) a solution of a tin salt, (3) a solution of a noble metal salt, and (4) a solution of a reducible salt of nickel or copper. The polyolefin is then electroplated with a metal film in accordance with conventional practice. Alternatively, the polyolefin may be vacuum metallized by any known technique.

At this time, we are not certain as to the mechanism by which the pretreatment step of this invention improves the bond strength of the electroplated coating. At least two theories have been advanced to explain the surprising improvements obtained. One theory is that the material is stressed when molded and pretreatment in accordance with this invention relieves the stresses which cause poor metallizing characteristics. This would be a type of local or surface annealing action. The other theory is that the pretreatment causes the molded material to be more easily attacked by the mineral acid used in the conditioning step of the electroplating process. This permits better oxidation of the surface and the formation of polar groups for receiving the metal ions during the electroplating.

EMBODIMENTS OF THE INVENTION In order to demonstrate the benefits obtained by treating polyolefins according to the method described in this invention, a series of experiments were performed. The bond strength of the electroplate to the substrate was measured by a standard pull test. In this test, two parallel cuts are made in the metal about /:.-inch apart. An additional vertical cut is made between the two parallel cuts to form a tab and one end of the resulting tab is raised sufficiently to allow gripping by a tensile testing machine. The specimen is then placed in a tensile tester and the tab is pulled vertically from the surface of the polypropylene. The force required to pull the tab is measured as the bond strength.

The polyolefins treated according to this invention include polymers which contain a major proportion (i.e., greater than 50%) of an aliphatic olefin, having from 28 carbon atoms. Such polyolefins, therefore, include polyethylene, polypropylene, ethylene-propylene copolymers,

ethylene-butene-l copolymers, polybutene-l, poly (4- methylpentene-l), poly (3-methylbutene-l) and the like. The term polyolefin as used herein is, furthermore, intended to include copolymers of hydrocarbon monomers with copolymerizable polar monomers in which such functional monomers constitute a minor proportion of the co- EXAMPLE 1 A plaque 5" x 5" x 110 mil of crystalline polypropylene having a flow rate of 5.0 (ASTM D1238-62T) was electroplated in accordance with the method described above but without the use of the pretreatment which is disclosed and claimed herein. The polyolefin plaque was molded from a homopolymer polypropylene considered to be a general purpose type. The resulting electroplate had zero 4 bond strength. Four other plaques molded from the same crystalline polypropylene were pretreated by placing the plaques in a bath containing methyl ethyl ketone at a temperature of about 78 C. for various periods of time. After such pretreatment, the plaques were electroplated according to the method described above and it was found that the bond strengths were significantly improved. The results from this experiment and the ones following are presented in the table below.

EXAMPLE '2 The procedure of Example 1 was repeated but this time the pretreatment was performed using acetone at 50 C. rather than methyl ethyl ketone. Again, the bond strength greatly increased.

EXAMPLE 3 The procedure of Example 1 was repeated using copolymerized polypropylene-polyethylene (flow rate 4.0) plaques instead of homopolymerized polypropylene plaques. After plating in accordance with the method described above it was found that the electroplate blistered from the untreated plaque but that the treated plaques had relatively high bond strengths.

EXAMPLE 4 Crystalline polypropylene having a flow rate of about 4 and modified by the addition of 0.5 part by weight of Triton X-100, a commercially available nonionic surfactant of t-octylphenoxypolyethoxyethanol, 0.5 part of dilauryl thiodipropionate and 5 parts of TiO' was molded into 5" x 5" x mil plaques and treated in accordance with the procedure outlined in Example 1. In this case, the pretreatment time was varied as was the temperature at which the methyl ethyl ketone was maintained. Even without pretreatment, the modified polypropylene plaques molded from the modified polypropylene electroplatable formulation accepted the electroplate and had a fairly good bond strength. However, after pretreatment, it was found that there was some additional increase in bond strength.

Pretreatment time (minutes) The foregoing examples have illustrated the improved process for metallizing molded polyolefin articles in which the article is pretreated with a conditioner. It will be apparent that this specific, illustrated procedure can be equally employed with other plastics within the scope of the present invention and that the conditioner 'can also be employed in other electroplating procedures. The foregoing examples are, therefore, to be considered as illustrative of the invention and are not intended to limit the scope of the invention thereto, many variations and modifications of the procedures and materials employed in the examples being obvious to those skilled in the art.

What is claimed is:

1. In a process for the deposition of metallic coatings on a shaped polyolefin article, the improvement comprising pretreating said shaped article by contacting said shaped article with a ketone which is dispersible, emulsifi able or at least slightly soluble in water, wherein said contacting occurs at a temperature between about 20 C. and the boiling point of said ketone and the extent of said contacting being such that said ketone penetrates the surface of said polyolefin article and thereby relieves strains in the surface of said polyolefin article.

2. A process according to claim 1 wherein the deposition of metallic coatings is accomplished by electroplating or vacuum metallizing said shaped polyolefin article.

3. A process according to claim 1 wherein said polyolefin is polypropylene.

4. A process according to claim 1 wherein said ketone is selected from the group consisting of acetone and methyl ethyl ketone.

5. A process according to claim 1 wherein said contacting is performed with a ketone in a liquid state.

6. A process according to claim 1 wherein said contacting is performed with a ketone in a vapor state.

7. A process according to claim 1 wherein said pretreating is accomplished by contacting said shaped polyolefin article with a ketone having a boiling point less than about 140 C. for a period of from about one minute to about fifteen minutes and at a temperature of from about 20 C. to the temperature at which the ketone boils, and thereafter washing said article prior to treating it with a metallizing process.

8. A process according to claim 7 wherein said metallizing process is electroplating.

9. A process according to claim 8 wherein said pretreated polyolefin article is subsequently treated with (a) a strong mineral acid solution containing chromium trioxide or an alkali metal chromate; (b) a solution of tin salt;

(c) a solution of a noble metal salt; and (d) a solution of a reducible salt of a metal selected from the group consisting of nickel and copper; and wherein steps (a) through (d) are followed by elec- 10 troplating to deposit a film of metal on the surface of said shaped polyolefin article.

10. A process according to claim 7 wherein said polyolefin is polypropylene and said ketone is acetone.

References Cited UNITED STATES PATENTS JOHN H. MACK, Primary Examiner T. TUFARIELLO, Assistant Examiner US. Cl. X.R. 

